Electron discharge device



July 10, l956 G. w. FRANKs UAL 2,754,428

ELECTRON DISCHARGE DEVICE Filed Sept. 17, 1951 2 Sheets-$heet l AJ R n ns E o wo m s o n Ew M www o u Wm m ,m .m w mm m mw 5f@ 4 mw a m m m 7 .fa f I- .wam M o m .m M w e N .AVul JM um md n 6 El i. E w w 1 m M m M 62 M um m ad A Lf ..1 M f., .A10 f f f m v mi w mm A d July 10,1956 G. w.FRANKs ET Al.

ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 2 Filed sept. 17, 1951 MPM T HZmm z mf. m

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www mw m F m w mw m m m ATTORNTJ ELEC'rRoN DISCHARGE DEVICE George W.Franks, Lorton, and John N. Hamilton, Alexandria, Va.

Application September 17, 1951, Serial No. 247,012

3 Claims. (Cl. Z50-211) (Granted under Title 35, U. S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes, without payment to us ofany royalty thereon.

This invention relates to electron discharge devices generally and moreparticularly to a self-energizing, photosensitive electron dischargedevice and method of initiating and maintaining the discharge.

Electron discharge devices of the type contemplated herein usuallyembodying a source of electrons known as a cathode and an electronreceiving or collecting electrode commonly known as an anode. The usualdevice requires some form of external energy to energize the emission ofelectrons from the cathode surface. In its simplest form, this type ofelectron discharge device has only two elements, i. e., the anode andcathode, and is applicable as a detector or rectilier. For exemplarypurposes, the description of this invention will be limited to thesimplest form of the device, which is a detector or rectiier.

A conventional electronic rectier generally consists of the cathode,which is the source of electrons, and an anode or plate which is thecollector element, both of which are sealed within a housing or envelopecontaining either a gas or a high vacuum. A common type of conventionaldetector or rectier utilizes a heated element as the source ofelectrons. Thus, the conventional discharge device requires an externalsource of energy to provide a source of electrons.

It is an object of this invention to provide an electrical dischargedevice having an electron collector and a source of electrons that isactivated by electro-magnetic energy produced within the device.

lt is a further object of this invention to provide a photosensitivesource of electrons which may be activated to freely emit electrons byabsorption of electro-magnetic energy radiated from a luminescentelectron collector.

A still further object of this invention is to provide an electrondischarge device having a photo-sensitive source of electrons which isactivated by electro-magnetic energy radiated from a luminescentelectron collector initially rendered luminescent by electron activationresulting from ambient thermal energy activating the photosensitivesource without other excitation under normal operating conditions.

A still further object of this invention is to aid the emission ofelectrons from a photosensitive source to activate a luminescentelectron collector by providing a radioactive material in proximity tothe electron collector.

Briefly, in accordance with this invention, there is provided anelectron discharge device having a radiation absorbing cathode whichfreely emits electrons in response to absorption of electro-magneticradiated energy generated within the device and having anelectron-excitable radiating anode which radiates electro-magnetic wavescorresponding to the spectral response of the radiation absorbingcathode in response to bombardment by cathode electrons. The radiatinganode may be initially excited to radiate electro-magnetic energy bybombard- 2,754,428 Patented July 10, 1956 fic ' 2 ment from electronsemitted from the cathode due to ambient thermal energy without otherexcitation under normal operating conditions. In the alternative, thecatode electron emission may be aided in exciting the anode byintroduction of a radio-active material. The radiating anode andelectron emissive cathode are mounted in spaced relation within anenvelope and are each connected to externally accessible electricalconducting means extending from the envelope for connecting the cathodeand anode to an electrically energized circuit.

Fig. l illustrates the construction of the electron discharge device asembodied in a half-wave .rectiiier having a single anode and cathode;

Fig. 2 illustrates the electron discharge device as embodied in afull-wave rectifier having a pair of anodes and a corresponding cathode;

Figs. 3 and 4 are vertical sectional views taken along the lines 3 3 and4--4 in Figs. l and 2;

Fig. 5 is a schematic illustration of the half-wave rectifier of Fig. lconnected in an electrically energized circuit;

Fig. 6 is a further embodiment of the invention wherein the radio-activematerial is a separate element of the electron discharge device; and

Fig. 7 is a vertical sectional view taken on the line 7--7 of Fig. 6.

As hereinbefore indicated, an electron discharge device, whenexemplarily embodied as a rectiiier, consists of a source of electronsin the form of a cathode and a collector of electrons in the form of ananode or plate which are both sealed within a housing containing eithera gas or high vacuum. The cathode source of electrons in a conventionalrectifier is usually provided with an element heated directly orindirectly to initiate the emission of electrons. This inventioncontemplates the substitution of a photo-emissive or radiation-absorbingelement for the cathode of a conventional type discharge device whichdoes not require external electrical energy to initiate electronemission. With this substitution, if the photo-emissive cathode wereilluminated with external electro-magnetic radiation which matches theresponse characteristic of the photo-sensitive cathode surface, thedevice would function in a manner equivalent to a cold cathoderectifier. Thus, this invention further contemplates the elimination ofthe external electrical source of electro-magnetic radiation byproviding an electron collector or anode which is coated with a phosphorand which is capable of being excited to luminescence by electronbombardment, the luminous energy output of which is properly selected tomatch the spectral characteristic of the photo-emissive cathode andwhich has suitable persistence characteristics. This method ofexcitation may be termed cathodeluminescenceg that is, excitation byelectrons emitted from the photocathode.

The electron discharge device thus constructed is energized under normalconditions of temperature without the requirement of additionalradio-active elements or isotopes to excite the luminescent anodematerial to radiation. This results from the fact that, even undernormal operating conditions, there is an equilibrium condition ofconstant small emission due to ambient thermal energy acting on thecathode surface material and return of electrons from photosensitivecathodes of the type herein contemplated in accordance with Richardsonsequation:

I :Emission current density A and a=Constants depending upon theemitting substance and the state of its surface T :Temperature indegrees Kelvin s KzBoltzmanns constant (1.3l7 10*16 erg per degree perelectron) G=The Napierian base of logarithms This condition has givenrise to the suggestion vof an autophotoelectric effect. Regardless ofthe theory, however, this small emission current due to ambient thermalenergy is considered to be sucient under the influence of a suitablepotential to cause initial excitation of the luminescent material of theanodes; theluminous energy emitted thereby produces photo-emission atthe cathode resulting in the build-up of the current through therectifier or other discharge device as hereinafter to be more fullydescribed. v

There are numerous types of photo-emissive cathode surfaces which may beexcited to emit photo-electrons in accordance with the foregoingrequirements. The material used must meet, as a minimum, two basicrequirements which are, rst, that the photocathode should have a highsensitivity, i. e., a high value of micro-amperes per lumen, and,secondly, the response curve ofthe surface should match the light outputof the anode luminescent phosphor. The exact composition of thepreferred photo-emissive surface is not known; however, it has beenprepared by empirically mixing antimony, cesiurn, and sometimes oxygen,and is known to the trade asthe S-4 antimony-cesium surface anddescribed, for example, in the text Photo-Electricity by Zworykin andRamberg, published by Wiley in 1949 on pages 114-115. In accordance withthe preferred embodiment of this invention, the photo-emissive surfacemay be formed on the inside of a glass, metal, or quartz envelope in anywell-known manner of which the following is an example. The bulb isfirst exhausted in a conventional manner and baked at about 350 C. Afterthe bulb is cooled, antimony is evaporated on to the inner bulb surface,usually from a heated tungsten coil inside the bulb which containsantimony. There is an optimum thickness of the antimony layer which isof the order of one micron. The bulb is then heated to approximately150"Y C. and cesium vapor is liberated into the bulb until a maximumsensitivity is reached. The bulb is then allowed to cool, after whichseveral cycles of liberating cesium, followed by short baking of thebulb, is accomplished. It is understood that increased sensitivity. maybe achieved by liberating suitable quantities of oxygen into the bulb.In practice it has been found that the resulting surfaces quite oftenhave too high an ohmic resistance for proper functioning. In order toalleviate this condition, a highly conducting surface, such asevaporated silver, may be used as a base for the photo-emissive cathode.A high degree'of sensitivity is obtainable by using various alloys ofantimony, such as gold, silver, copper, and zinc.

As hereinbefore indicated, the selection of the luminescent anodematerial is determined so as to match the spectral response of theresultant high eiciency photocathode. It is generally understood thatthe highest degree of eiciency is obtainable in the ultra-violet end ofthe spectrum. However, in accordance with the preferred embodiment ofthis invention, it is preferred to use a spectral response in thevisible range. Y

The emission of luminous energy from matter under the influence of anexciting agent when in the visible range, is termed uorescence. sistsafter the removal of the exciting agent, the action is termedphosphorescence. The method of excitation utilized in the preferredembodiment of thisrinvention has been previously termedcathode-luminescence, i. e., the excitation of the radiation-emittingsurface by electrons emitted from the photo-emissive surface of acathode. There are numerous substances which exhibit the proper degreeof fluorescence, and the selection of the' anode luminescent materialfor the purposes of this invention is determined in accordance with twomajor requirements, namely, the high efficiency of fluorescence whenexcited When the emission peri electron bombardment, and the radiationspectra matching as nearly as possible the response characteristics ofthe photo-emissive cathode. Any phosphor having these characteristicsmay be used, however in practice, it has been found that the phosphorcomposition ZnS:Ag exhibits the correct conditions to meet theserequirements .when used with an S-4 surface as previously described.Additional continuous emission of light from the phosphor may beobtained if necessary or desirable by combining it with any of severalradioactive elements or isotopes, such as radium. The phosphor coatingsor screens may be prepared by any of several well-known methods, theusual method involving preparation of a suspension in distilled water towhich a binding agent is added, with subsequent removalV of thesuspended particles by settling, centrifuging, electrostatic depositionand spraying.

Referring now to Fig. 1, there is illustrated the preferred embodimentof the electron discharge device of this invention as physicallyexemplified in a half-wave rectier construction. The device includes aninsulating base 1 of suitable material such as porcelain or the likewhich base has a raised portion in the form of a boss 2 whichl has acentral recess therein for receiving Vand supporting the anode 3. Theanode 3 is provided with a phosphor coated surface 4, which may compriseradio-active material, as hereinbefore described, and which may bedeposited on or applied to the anode surface in accordance with theseveral methods hereinbefore described or suggested. Although it isunderstood that the discharge ldevice will operate as a half-waverectier in the construction of Fig. l without the inclusion of a screen,the preferred embodiment is showrnrprovided with Van electroncontrolling screen 5, which may be in the form of a suitablywoven wiremesh mounted in concentric relation aroundy the anode 3 and connected inthe ultimate circuit so as to Vremain positive with respect to thecathode to reduce bombardment of the cathode by electrons when thecathode is positive with respect to the anode Von the reverse halfcycle. Other modes of operation may be used to accomplish a similarresult, such as connecting the screen directly to the anode toV lessenthe extent of electron bombardment of the cathode by creating anelectron free eld between Vthe screen and anode. The assembly of theinsulating support, anode, and screen is inclosed within avacuum in asuitable glass, metal, or quartz envelope 6` which is coated onits-inner surface with a suitable photo-emissive or sensitized cathodematerial 7 in accordance with the methods hereinbefore described. Theenvelope 6 is provided with an opening in one side thereof having aconnecting cap 8 which cap is connected to a lead-in conductor 9 leadinginto electrical contact with the photo-emissive cathode surface withinthe envelope. Suitable electrical conductors Mi and 11 are shownVconnected to the anode and screen respectively and extending from thebase of the envelope to be externallyv accessiblefor connection to asuitable electrically energized circuit as exemplarily illustrated inFig.

5 of the drawings.

The embodiment illustrated in Fig. 2 exemplifies the physical embodimentof the electron discharge device of this invention inthe form of afull-wave rectifier having two anodes 12 and 13 mounted in spacedrelation within Ythe envelope in place of the single anode of Fig. 1. In

be unnecessary since the potential of one or the other of the anodeswould be equal to or positive with respect to `the cathode at all times.l

v,In lieu of providing radio-active material as a component of the anodecoating, a separate electrode providing radio-active material may beused, as recited elsewhere in this specitication and illustrated inFigs. 6 and 7. The structure of this embodiment is similar to that shownin Fig. l with the addition of the electrode 14 which providesradio-active material.

Thus, the combination of a photosensitive cathode, which has a constantsmall electron emission under normal operating conditions due to ambientthermal energy, mounted in spaced relation from an anode which may beactivated to luminescence by virtue of electron bombardment resultingfrom electron emission from the photo-emissive cathode or, as furtheraided by the interposition of a suitable radio-active material in theregion of the luminescent anode, results in an electron dis- Wchargedevicernot requiring external electrical sources sidered preferredembodiments of this invention, it will be obvious to those skilled inthe art that changes and modifications, particularly in respect to thenumber of electron controlling or activated elements and theirarrangement and application in electrical circuits, may be made withoutdeparting from the scope of this invention as defined by the appendedclaims.

Having thus described our invention, what we claim as new and wish tosecure by Letters Patent is:

1. An electron discharge device comprising in combination, anelectron-excitable electro-magnetic radiating element, a radiationabsorbing element which at low ambient temperatures freely emitselectrons in response to such absorption, a radio-active element mountedin proximity to said electron-excitable electro-magnetic radiatingelement for exciting said radiating element, means mounting saidradiating element, said radiation absorbing element and saidradio-active element in mutually spaced relation, an envelope enclosingsaid mutually spaced elements, externally accessible electricalconductor means extending from the envelope for connecting saidradiating and radiation absorbing elements to an electrically energizedcircuit.

2. A structure as recited in claim 1 including an electron controllingscreen element concentrically arranged with said electron-excitableelectro-magnetic radiating element and intermediate said radiatingelement and said radiation absorbing element, and externally accessibleelectrical conductor means extending from the envelope for connectingthe screen to an electron attracting potential in an electricallyenergized circuit.

3. An electron discharge device comprising in combination, an anodehaving a phosphor coating capable of being excited by electronbombardment to luminescence of predetermined spectral response, aradio-active element mounted in mutually spaced relation from saidanode, a woven wire mesh screen formed to concentrically surround saidanode in spaced relation therefrom, an insulating support includingmeans to support said anode, said radio-active element and said screenin mutually spaced relation, a glass envelope inclosing the support,anode, radio-active element, and screen in a vacuum, said glass envelopehaving an inner photo-emissive coating with an absorption characteristicmatching the spectral response of said anode, said photo-emissivecoating being activated by ambient thermal energy and exhibiting aconstant electron emission under normal operating conditions withoutexternal electrical excitation, said raido-active element insuringexcitation of said anode surface in the absence of ambient thermalenergy sucient to activate said photo-emissive surface, a source ofelectrical potential, and externally accessible electrical conductorsextending from the envelope connecting said anode and photo-emissivecoating across said source of electrical potential and said screen to anelectron attracting electrical potential.

References Cited in the tile of this patent UNITED STATES PATENTS1,145,735 Ainsworth July 6, 1915 1,173,110 Junghans Feb. 22, 19161,775,588 Cohn Sept. 9, 1930 1,788,553 Thomas Jan. 13, 1931 1,917,854Rentschler July 11, 1933 1,965,849 McIlvaine July l0, 1934 2,039,134Waldschmidt Apr. 28, 1936 2,092,814 Schaiernicht Sept. 14, 19372,206,387 Bruche July 2, 1940 2,218,340 Maurer Oct. 15, 1940 2,258,294Lubszynski et al Oct. 7, 1941 FOREIGN PATENTS 499,661 Great Britain Jan.26, 1939

